The present invention relates to radiation-sensitive mixtures which are particularly suitable for preparing photoresist stencils and printing plates. The present invention relates in particular to positive-working radiation-or light-sensitive mixtures based on combinations of photolytically activatable acid donors with acid-cleavable compounds and water-insoluble binders which are soluble in aqueous alkaline solutions.
Positive-working light-sensitive mixtures of the aforesaid constituents, i.e., mixtures whose copying layer becomes soluble in the exposed areas are known. In the vast majority of cases, these mixtures contain phenol-formaldehyde condensation products, in particular novolaks as alkali-soluble binders. Also used for this purpose are copolymers of maleic anhydride and styrene, of vinyl acetate and crotonic acid, of alkyl methacrylate, butadiene and methacrylic acid (German Offenlegungsschrift No. 3,107,526) and polymers of vinylphenol (German Offenlengungsschfift No. 3,309,222). These polymers are evidently not preferred, since practically all known commecial products contain novolaks. Nevertheless, novolaks used as binders present disadvantages for certain purposes.
For use as binders in dry resist coatings, novolaks are too brittle because of their low molecular weight, so that a novolak-containing layer frequently breaks during processing. Layer breakage can occur, for example, in the course of exposure when the layer is in contact with a mask, or in the course of lamination when the layer is applied as a dry resist to a base material surface. Layer brittleness, which is particularly noticeable in the case of the higher layer thicknesses preferred for dry resist materials, is caused by a plurality of factors. First, the incorporation of aromatic groups in the main chain substantially restricts the mobility of the main chain, which effect manifests itself in a high glass transition temperature and low flexibility. Secondly, due to the polyfunctionality of the compounds used, the condensation of phenols or cresols with formaldehyde under acid conditions, which is customarily used for the synthesis of novolaks, leads only to products having a low molecular weight an/or a high degree of branching or crosslinking.
Because of these novolak-specific disadvantages, the film-forming capacity and flexibility of such compounds is too low to permit their use, in combination with light-sensitive systems, in preparing dry resist films which can be satisfactory processed from a roll, without breaking of the layer or delaminating of resist to the protective film.
Novolak brittleness is also the reason for processing problems encountered when using these binders in microelectronics. During exposure which entails intimate contact with a mask, breaks or cracks often appear in the layer.
European patent application No. 0 042 562 discloses a corresponding light-sensitive mixture which, to increase the flexibility of the layer and to improve other properties, contains another polymer, for example a polyurethane, a polyvinyl alkyl ether, a polyalkyl acrylate or a hydrogenated colophony derivative, in addition to the alkali-soluble novolak. But the addition of these resins, which have a different solubility behavior than novolaks, has an adverse effect on other properties, such as developability, acid and alkali resistance of the image stencil.
It has therefore been proposed to replace the novolak in positive-working layers with another alkali-soluble binder. Polyvinylphenol is disclosed for this purpose in German Patentschrift No. 2,322,230, that is, as a novolak replacement, in layers that contain o-quinonediazides. Although the plasma resistance and the adhesion to silicon surfaces are basically comparable to novolaks, unexposed photoresist layers based on this polymer are excessively eroded in the course of development. It is true that the degree of erosion decreases with increasing molecular weight, but in the best cases (molecular weight .congruent.25,000) it is still higher than in the case of novolak-based layers by a factor of approximately 100 (according to T. R. Pampalone, Solid State Technology, June, 1984 at page 119). A further disadvantage is the preparatively expensive synthesis and the low polymerization tendency of monomeric vinylphenol.
Japanese Kokai No. 51/36,129 describes the use as binders of polymers based on hydroquinone monomethacrylate and dihydroxynaphthalene monomethacrylate in naphthoquinonediazide layers.
All these mixtures with o-quinonediazides as light-sensitive compounds have the disadvantage, compared with mixtures with the acid-cleavable compounds mentioned above, of a distinctly lower light sensitivity. This is particularly evident in uses where higher layer thicknesses are required, for example, in dry resist processes.